OpenSSH Security Advisory (adv.iss)

[Markus Friedl]

1. Versions affected:

        All versions of OpenSSH's sshd between 2.9.9 and 3.3
        contain an input validation error that can result in
        an integer overflow and privilege escalation.

        OpenSSH 3.4 and later are not affected.

        OpenSSH 3.2 and later prevent privilege escalation
        if UsePrivilegeSeparation is enabled in sshd_config.
        OpenSSH 3.3 enables UsePrivilegeSeparation by
        default.

        Although OpenSSH 2.9 and earlier are not affected
        upgrading to OpenSSH 3.4 is recommended, because
        OpenSSH 3.4 adds checks for a class of potential bugs.

2. Impact:

        This bug can be exploited remotely if
        ChallengeResponseAuthentication is enabled in sshd_config.

	Affected are at least systems supporting
	s/key over SSH protocol version 2 (OpenBSD, FreeBSD
	and NetBSD as well as other systems supporting
	s/key with SSH).  Exploitablitly of systems
	using PAM in combination has not been verified.

3. Short-Term Solution:
	
        Disable ChallengeResponseAuthentication in sshd_config.

	or

        Enable UsePrivilegeSeparation in sshd_config.

4. Solution:

	Upgrade to OpenSSH 3.4 or apply the following patches.

5. Credits:

	ISS.

Appendix:

A:

Index: auth2-chall.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/auth2-chall.c,v
retrieving revision 1.18
diff -u -r1.18 auth2-chall.c
--- auth2-chall.c	19 Jun 2002 00:27:55 -0000	1.18
+++ auth2-chall.c	26 Jun 2002 09:37:03 -0000
@@ -256,6 +256,8 @@
 
 	authctxt->postponed = 0;	/* reset */
 	nresp = packet_get_int();
+	if (nresp > 100)
+		fatal("input_userauth_info_response: nresp too big %u", nresp);
 	if (nresp > 0) {
 		response = xmalloc(nresp * sizeof(char*));
 		for (i = 0; i < nresp; i++)

B:

Index: auth2-pam.c
===================================================================
RCS file: /var/cvs/openssh/auth2-pam.c,v
retrieving revision 1.12
diff -u -r1.12 auth2-pam.c
--- auth2-pam.c	22 Jan 2002 12:43:13 -0000	1.12
+++ auth2-pam.c	26 Jun 2002 10:12:31 -0000
@@ -140,6 +140,15 @@
 	nresp = packet_get_int();	/* Number of responses. */
 	debug("got %d responses", nresp);
 
+
+	if (nresp != context_pam2.num_expected)
+		fatal("%s: Received incorrect number of responses "
+		    "(expected %u, received %u)", __func__, nresp,
+		    context_pam2.num_expected);
+
+	if (nresp > 100)
+		fatal("%s: too many replies", __func__);
+
 	for (i = 0; i < nresp; i++) {
 		int j = context_pam2.prompts[i];